1. Field of the Invention
The present invention relates to a wireless communication apparatus, an antenna calibration method, and a program.
2. Description of the Related Art
Communication technology to achieve high throughput wireless data transmission includes multi-antenna technology that uses a wireless communication apparatus having a plurality of antennas. Adaptive array antenna is known as an example of the multi-antenna technology, and multi-input multi-output (MIMO) communication system is known as an application of the multi-antenna technology.
In the adaptive array antenna, each transmission signal is multiplied by an appropriate weighting factor and transmitted from each antenna, and a signal received by each antenna is multiplied by an appropriate weighting factor and then the weighted signals are combined, thereby controlling the directivity patterns of transmission and reception of the array antenna as a whole. Further, in the MIMO system, both a transmitter and a receiver include a plurality of antennas, and the transmitter transmits a plurality of signal streams using spatial multiplexing, and the receiver separates a received signal into a plurality of signal streams by signal processing.
In any of the multi-antenna technology, the directivity patterns of transmission and reception of the communication apparatus are brought into agreement with each other, so that the main lobe is directed in a desired direction in both transmission and reception thereby achieving the high throughput data transmission. The agreement of the directivity patterns of transmission and reception indicates the establishment of reversibility.
However, in the adaptive array antenna, the reversibility is lost due to characteristics of each element constituting transmitting and receiving systems, and even if the weighting factor calculated at the time of reception is used at the time of transmission, the directivity patterns of transmission and reception cease to agree. Therefore, a correction value is calculated and the weighting factor is corrected using the correction value in advance as antenna calibration, so that the directivity patterns of transmission and reception are brought into agreement. Further, in the MIMO communication apparatus, because a transfer function of a spatial channel and a transfer function of an analog section in the apparatus are recognized altogether as a channel matrix, the reversibility is lost if a transfer function is different between an analog transmitting section and an analog receiving section. Therefore, the antenna calibration is performed in advance so as to bring the directivity patterns of transmission and reception into agreement.
An example of the case where it is necessary to establish the reversibility is when a transmitter transmits a plurality of signal streams using a weighting factor in the MIMO communication apparatus. The use of the weighting factor by the transmitter enables optimization of spatial multiplexing and spatial separation in both of the transmitter and the receiver. In this case, the transmitter acquires a transfer function and calculates a weighting factor based on the reception status of a reference signal that is transmitted from the receiver with use of the reversibility that is established in the spatial channel between the transmitter and the receiver. However, even if the reversibility in the spatial channel is established, (branch) imbalance exists in the characteristics of a transfer function between each analog transmitting section and each analog receiving section in a plurality of transmitting and receiving branches that constitute the communication apparatus due to an error in manufacturing or the like. Therefore, it is necessary to perform antenna calibration that corrects the branch imbalance in order to calculate a weighting factor based on the reception status of a reference signal transmitted from the receiver.
The antenna calibration is generally performed in the following steps as described in Japanese Unexamined Patent Application Publication No. 2007-116489, for example. In the antenna calibration according to the related art, first, the branch 0 is set to the transmitting end, and the branches 1 and 2 are set to the receiving end, and then a reference signal transmitted from the branch 0 is received by the branches 1 and 2, and a loopback transfer function in each path is acquired from the reception status of the reference signal (first acquisition step). Next, the branch 1 is set to the transmitting end, and the branch 0 is set to the receiving end, and then a reference signal transmitted from the branch 1 is received by the branch 0, and a loopback transfer function in the path is acquired from the reception status of the reference signal (second acquisition step). Further, the branch 2 is set to the transmitting end, and the branch 0 is set to the receiving end, and then a reference signal transmitted from the branch 2 is received by the branch 0, and a loopback transfer function in the path is acquired from the reception status of the reference signal (third acquisition step). After that, a calibration factor as a correction value is calculated based on the loopback transfer functions that are acquired in the all paths, and a weighting factor is corrected using the correction value. The reference signal is a known signal that is transmitted and received in the path including the spatial channel and the analog transmitting and receiving sections in order to acquire the loopback transfer function.